1. Field of the Invention
The present invention pertains to an artificial heart valve, and, more particularly, pertains to a mechanical artificial heart valve for use in humans.
2. Description of the Prior Art
There are two basic types of artificial heart valves, biological and mechanical. The medical indications for heart valve replacement are the same for both types. Examples include calcified, congenitally bicuspid aortic valves, rheumatic valvular disease, and rupture of a chordae tendinae.
Biological valves are either harvested from animals, such as pigs (porcine valves) or constructed from animal parts, such as cow pericardium (bovine valves). The primary advantage of these valves is that they do not require long term anticoagulation therapy with drugs like warfarin. These valves are used frequently in situations where anticoagulants are contraindicated, such as pregnant patients, or when medical follow-up may be tedious, such as in underdeveloped countries.
Mechanical valves are made of materials, such as pyrolitic carbon and titanium. In general, mechanical valves are very durable, with service lives expected to exceed the life expectancy of the patient. All mechanical valves in current use require anticoagulation. And, if there are no contraindications to such medications, mechanical heart valves are usually preferred by most physicians.
The mechanical valves commercially available at present, for example, are based on disclosures in U.S. Pat. No. 4,276,658-Hanson et al dated July 7, 1981; U.S. Pat. No. Re. 31,040-Possis dated Sept. 28, 1982; and U.S. Pat. No. 4,689,046-Bokros et al dated Aug. 25, 1987. All of these mechanical valves utilize the same essential features. That is, they consist of an orifice (an annular shaped frame) with one or more flat leaflets, which are free to rotate, within certain limitations, within the orifice. The restrained motion of these leaflets causes the flow through the orifice to be essentially unidirectional, which mimics the natural function of normal (native) heart valves.
The unidirectional flow characteristic is the primary function of a heart valve, both native and mechanical. Native valves, however, have evolved to a form that also minimizes the work load on the heart. This is accomplished by streamlining the shape of the leaflets to minimize the amount of turbulence (separation of flow) in the wake of the leaflets and to minimize the amount of backflow (regurgitation) during the closure of the leaflets.
Flat leaflets are inherently subject to increased separation effects and, therefore, lead to wasted energy and extra work load on the heart. This has been disclosed in an article "Design Considerations for the Omniscience Pivoting Disc Prosthetic Heart Valve" by Dr. Thomas H. Reif in the International Journal of Artificial Organs (1983), vol. 6, no. 3, pp. 131-138. This article demonstrated that separation effects increase as the degree of opening of the leaflet(s) decreases. The opening angle is mathematically defined by an increase in the angle of attack. It was further demonstrated that these separation effects could be reduced, when compared to flat leaflets at the same angle of attack, by using a curved leaflet with the leading edge (during the forward flow phase) parallel to the flow at the inlet of the orifice. A single curved leaflet mechanical valve disclosed in U.S. Pat. No. 4,240,161-Huffstutler et al dated Dec. 23, 1980, is based on this concept. Such a configuration does improve separation effects, however, it does not eliminate them. Furthermore, the outlet flow is malaligned due to the curvature at the trailing edge.
U.S. Pat. No. 4,775,378-Knock et al dated Oct. 4, 1988, discloses a further improvement of this concept by utilizing an S-shaped leaflet. In this configuration both the leading and trailinq edges of the leaflets are aligned parallel to the forward flow. Little attention was given to separation effects, as the chord of the leading edge airfoil is longer than the chord of the trailing edge airfoil. Dr. Hermann Schlichting discloses in his text Boundary-Layer Theory, 7th ed., McGraw-Hill Co., New York, 1979, pp. 168-173, that the flow over right circular cylinders rapidly separates on the side of the adverse pressure gradient (region of decelerating flow). Also, this effect is strongly dependent on the ratio of inertia to viscous effects (mathematically defined as the Reynolds number). That is, the separation effects being less important at Reynolds numbers (based on the radius of the cylinder) of greater than 3.times.10.sup.5. Physiological flows, however, rarely develop Reynolds numbers (based on the approximate radius of curvature of the leading edge airfoil) in excess of 1.5.times.10.sup.4. Thus, separation effects are of great importance. Configuring the leaflets, such that the ratio of the chord of the leading edge airfoil is greater than one (1) is not desirable. This is because sufficient time is given for the fluid particles to separate from the leaflet prior to the change in curvature.
All mechanical heart valves have a certain amount of backflow or regurgitation. This regurgitation can be broken down into two components, during each cardiac cycle. One component is the closing reflux volume, which occurs as the leaflets close and sweep some of the fluid through the orifice in the direction opposite to the forward flow. It is an elementary task, using the Theorem of Pappus, to show that the closing reflux volume is proportional to the angle of attack. As the angle of attack becomes smaller, the closing reflux volume must increase. Further, slight modifications in the shape of the leaflets can significantly influence the closing reflux volume. Leaflets which are not planar may protrude into the orifice, so as to further reduce the closing reflux volume. The valve disclosed by U.S. Pat. No. 4,775,378-Knock dated Oct. 4, 1988, has such a feature. However, because the ratio of the chord of the leading edge airfoil to the chord of the trailing edge airfoil is greater than one (1), this effect is minimized.
The second component of regurgitation is leakage, which occurs because of the imperfect seal between the leaflets and the orifice, when the leaflets are in the closed position. Differences in the regurgitation characteristics of mechanical heart valves have long been noted. However, these differences have generally been accepted as consequences of the angle of attack of the leaflets and the size of the clearance space between the leaflets and the orifice.
In summary, there are several disadvantages to the current or prior art design configuration of the leaflets of mechanical heart valves. All designs demonstrate marked separation effects and at least some regurgitation. Both of these factors lead to increased work load on the heart, as easily demonstrated by fundamental principles of fluid mechanics.